The invention relates to a method for producing at least one signal (oscillation signal), which indicates an oscillation in an electrical power supply system.
German Laid-Open Specification DE 195 03 626 A1 describes a method of identifying an oscillation. In this method, once a memory element has been set, further impedance values are checked to determine whether the oscillation that has been found is still continuing, by determining the rate of change of the magnitude of respectively successive impedance values. If it is found that the rate of change is above a limit value, this identifies that the oscillation has stopped, and the memory element is reset. It is difficult to define such a limit value, particularly when a large number of generators are connected in the power supply networks, and complex oscillations can thus occur.
The invention relates to a method for producing at least one signal (oscillation signal), which indicates an oscillation in an electrical power supply system, in which method the phase current and the phase voltage are in each case sampled from at least one phase of the power supply system, forming phase current and phase voltage sample values, impedance values are formed from the phase current and phase voltage sample values, the impedance values are monitored for the presence of any oscillation and, if an oscillation is identified, at least one memory element is set, and the oscillation signal is output at its output, after setting the memory element, further impedance values are checked to determine whether the oscillation that has been found is still continuing, the memory element remains uninfluenced if the oscillation continues, and the memory element is reset if the oscillation has stopped.
The invention specifies a method of detecting the oscillation behavior of an electrical power supply system in a safe and reliable manner.
In one embodiment of the invention, there is a method that checks the impedance values and makes use of an oscillation model which is formed from previous impedance values associated with the oscillation, or is from variables which are dependent on these impedance values. A check is then carried out to determine whether a further impedance value formed at that time or a variable which is dependent on the further impedance value differs from the oscillation model, and any occurrence of a further impedance value or of a variable dependent on this impedance value which differs from the oscillation model is assessed as the oscillation having stopped.
One advantage of the method according to the invention is that the oscillation model allows even complex oscillations to be described, and it is thus possible to identify that the oscillation has stopped with a high level of reliability even in the case of such complex oscillations
The oscillation model can advantageously be determined by means of a least squares estimation method. This estimation method allows a mathematical oscillation model to be produced from successive impedance values which have been formed after the setting of the memory element, that is to say after the start of the oscillation.
A function in the form f(x)=ax3+bx2+cx+d with the parameters a, b, c and d can be used as the model rule or the oscillation model, in which one or more parameters can be defined to be zero before the start of the estimation method. First, second or third order power functions can be used as the model rule. Furthermore, a sum of sine and cosine functions, which decay with time, can be used as the model rule for the oscillation model. These model rules make it possible to describe even complex oscillations mathematically.
The oscillation model can be formed directly for the determined impedance values of the oscillation, or else for variables dependent on these impedance values. Resistance values. R, reactance values X, time derivative values dZ/dt of the impedance, time derivative values dR/dt of a resistance or time derivative values dX/dt of a reactance can be used as dependent variables. Choice of the most suitable variable for the oscillation model makes it possible to determine with a high level of reliability that the oscillation has stopped, with the choice of the variable being dependent on the individual system configuration of the electrical power supply system.
In one advantageous embodiment of the invention, positive phase sequence system impedance values can be formed from the phase current and phase voltage sample values, and a common memory element can be provided, and a common oscillation signal can be produced, for the phases in the power supply system. This variant can be used when the aim is to make a statement relating to any oscillation occurring at the same time in all the phases in the power supply system.
In a further embodiment of the method according to the invention, phase impedance values are formed from the phase current and phase voltage sample values of each phase of the power supply system to be investigated for oscillation, and a dedicated memory element is provided, and a dedicated oscillation signal is produced, for each of these phases. In this embodiment, the oscillation response of each individual phase in the power supply system can be investigated separately. That is, both the starting and the stopping of an oscillation are identified. This is particularly advantageous when oscillations occur in a single phase, but not in all the phases, in the power supply system. Oscillations such as these frequently occur in the case of so-called single-pole pauses in high-voltage systems. Single-pole pauses are produced by single-pole conductor ground faults, which can be expected frequently in high-voltage systems, and in which an arc is struck between one conductor and ground. In this type of fault, a single-pole pause is produced. That is, the single phase in which the single-pole conductor-ground fault has occurred is switched off briefly. The arc is thus quenched, and the fault is frequently corrected. Switching off a single pole of one phase can result in oscillations occurring in the remaining phases which are not switched off. These oscillations cannot be identified, for example, by monitoring the positive phase sequence system impedance values, since positive phase sequence system impedance values are formed when sample values are available for the phases in the power supply system. In the case of a single-pole pause, it is advantageous to be able to produce a dedicated oscillation signal for each phase in the power supply system. During the single-pole pause, this oscillation signal is produced for those phases which are not switched off. The oscillation behavior of the power supply system can thus be determined individually for each phase, and independently of the state of the other phases.
The phase impedance values of the individual phases in the electrical power supply system can, for example, be formed by, in order to form the phase impedance values,
a variable U_re including the real part of the phase voltage sample values, a variable U_im including the imaginary part of the phase voltage sample values, a variable I_re including the real part of the phase current sample values and a variable I_im including the imaginary part of the phase current sample values being formed from the phase current and phase voltage sample values (i, u) for each phase,
a phase real power variable P being determined from P=U_rexc2x7I_rexe2x88x92U_imxc2x7I_im,
a phase wattless component variable Q being determined from Q=U_imxc2x7I_re+U_imxc2x7I_re,
a squared phase current amplitude variable I2 being determined from I2=I_rexc2x7I_re+I_imxc2x7I_im,
system-frequency components in each case being removed by means of least squares estimation method from the phase real power variable P, from the phase wattless component variable Q and from the squared phase current amplitude variable I2, and
phase resistance values R being determined from R=P/I2 and phase reactance values, X being determined from X=Q/I2, and phase impedance values Z=R+jX being thus determined.
When forming the phase impedance values, it is advantageous to remove the system frequency components (for example 50 Hz components) from the phase real power variable P, from the phase wattless component variable Q and from the squared phase current amplitude variable I2 by means a least squares estimation method in each case. Such system frequency interference components would adversely affect the evaluation of the phase impedance values determined from these variables.